An MRI apparatus comprises a static magnetic field generating device for generating a homogeneous static magnetic field in an imaging space, a gradient magnetic field coil for generating a gradient magnetic field, and a high-frequency coil, so as to obtain images effective for medical diagnosis by applying a high-frequency magnetic field from the high-frequency coil to an examining part of an object to be examined placed in a uniform static magnetic field space, detecting a nuclear magnetic resonance (hereinafter referred to as NMR) signal produced from the examining part and generating an image based on the detected signals. The gradient magnetic field coil applies, in the imaging space, to the gradient magnetic field wherein the magnetic intensity is varied in orthogonal 3-axis directions, in order to provide positional information to an MRI signal.
Commonly-known static magnetic field generating devices are such as the cylindrical magnetic structure type wherein the inside thereof is an imaging place or the open structure type wherein a pair of magnets are arranged above and below across an imaging space. The open-type static magnetic field generating device comprises a pair of magnets disposed to face each other while being supported by one or two support pillars, as disclosed in Patent Document 1. By such structure, the open-type static magnetic field generating device has wide opening in the direction where there is no support pillar, whereby moderating the sense of confinement and improving access performance of doctors or assistants to an object to be examined.
While the cylindrical-type static magnetic field generating device has a single-piece construction wherein an imaging space is surrounded 360° by a magnet whereby distortion of magnetic flux in the imaging space is hardly generated structurally, an open-type static magnetic field generating device has a tendency to easily generate distortion of magnetic flux since the magnet is divided in order to obtain openness. For this reason, the open-type static magnetic field generating devices are designed with careful attention to their construction in reducing distortion of magnetic flux to a minimum, due to the fact that high uniformity of static magnetic field is demanded.
Regardless of the static magnetic field generating device of an MRI apparatus being the cylindrical-type or the open-type, the problem still remains that a gradient magnetic field coil structure vibrates attributed to driving of a gradient magnetic field coil in a static magnetic field.
More specifically, when a current is applied on a gradient magnetic field coil disposed in a static magnetic field, Lorentz force acts on a coil conductor and a gradient magnetic field coil structure is oscillated due to the Lorentz force.
If the open-type magnet is the permanent magnet type, the vibration of the gradient magnetic field coil structure due to Lorentz force merely stays within the vibration in the coil structure which gives comparatively small influence since the supporting part of the gradient magnetic field coil has a rigid body such as a pole piece or an iron yoke. However, if the open-type magnet is a superconducting magnet type, since the supporting part of the gradient magnetic field coil has a flexible body such as a cryostat, the vibration affects not only the cryostat but also the superconducting coil in the cryostat whereby generating not only the vibration and noise attributed to the vibration in the gradient magnetic field coil but also nonuniformity or fluctuation of static magnetic field.
The technique for reducing noise attributed to vibration of a gradient magnetic field coil in an open-type MRI apparatus is suggested in Patent Document 1, which is to attach piezoelectric elements on the surface of the gradient magnetic field coil and apply a voltage to the piezoelectric elements in a predetermined timing based on drive information of the gradient magnetic field coil so as to cancel the vibration of the gradient magnetic field coil.    Patent Document 1: JP-A-H9-308617